This invention relates to a pressure regulator suitable for use in regulating the outlet pressure of a low pressure, transfer pump of a fuel system for a compression ignition internal combustion engine.
It is known to use, as the transfer pump for use in a fuel system, a vane pump which is arranged to operate at a speed associated with engine speed. The vane pump may be provided with a regulator of the type illustrated in FIG. 1, comprising a sleeve 10 within which a piston member 12 is slidable. The piston member 12 is biased by means of a spring 14 and is moveable under the action of the fuel pressure at the outlet of the transfer pump. When the piston member 12 is in the position shown in FIG. 1, the piston member 12 closes an outlet 16 defined within the sleeve 10 to prevent fuel flowing through the outlet 16 to the inlet side of the transfer pump. The piston member 12 is moveable within the sleeve 10 to a position in which the outlet 16 is open to allow fuel to return to the inlet side of the transfer pump.
The degree to which the outlet 16 is opened depends upon the magnitude of the pressure applied to the piston member 12. By appropriate selection of the size of the piston member 12 and the spring 14, the regulator can be set in such a manner that the outlet pressure of the transfer pump, as regulated by the pressure regulator, is related to the engine and pump speed in a desired manner.
The pressure at the outlet of the transfer pump, as modified by the pressure regulator, is used to control various aspects of fuel system operation. For example, in some arrangements the pressure at the outlet of the transfer pump, as modified by the pressure regulator, is used to adjust the maximum fuelling level of a high pressure fuel pump with which the transfer pump is associated. However, it has been found that setting of such transfer pumps is difficult as the relationship between the outlet pressure and speed varies from pump to pump as a result of production variations.
It is an object of the present invention to remove or alleviate this problem.
By way of background to the present invention, CH 690 191 A5 describes a pressure relief valve for relieving fluid pressure within a load chamber. The pressure relief valve is of the type having a valve needle biased towards a valve seating by means of a spring load. The valve needle is caused to lift away from the valve seating in the event that fluid pressure within the load chamber exceeds a predetermined amount, thereby opening communication between the load chamber and a low pressure drain. The pressure relief valve also includes a piston which is exposed to fluid pressure within a control chamber and which is engageable with a movable piece to alter the spring load acting on the valve needle. By varying fluid pressure within the control chamber, the fluid pressure at which the valve needle is caused to lift from its seating can be varied. The pressure relief valve is of the xe2x80x9cplug-in-holexe2x80x9d type in which the valve needle extends into an opening in an insert which defines the valve seating. If fluid pressure within the load chamber is less than the predetermined relief pressure, the pressure relief valve is in a plugged position in which the valve needle extends into the opening and seats against the valve seating to completely close the opening, closing communication between the load chamber and the low pressure drain. If fluid pressure within the load chamber exceeds the predetermined relief pressure, the pressure relief valve is opened and the valve needle lifts off the valve seating to relieve pressure in the load chamber.
According to a first aspect of the present invention, there is provided a pressure regulator for use in regulating the outlet pressure of a pump for use in an engine, the pressure regulator comprising a piston member which is moveable within a bore provided in a regulator housing with respect to an outlet opening under the influence of fluid pressure acting on the piston member to control the degree by which the outlet opening is obscured, the pressure regulator comprising a biasing arrangement for applying a biasing force to the piston member which opposes the pressure of fluid acting on the piston member, wherein the biasing arrangement comprises a moveable abutment member which is cooperable with a stop member so as to terminate movement of the abutment member when the abutment member moves into engagement with the stop member, in use, thereby to vary the rate at which the displacement of the piston member varies with fluid pressure acting on the piston member, the stop member having a pre-set fixed position relative to the regulator housing.
Preferably, the biasing arrangement comprises a spring assembly comprising first and second spring means.
Conveniently, the first and second spring means take the form of first and second compression springs.
The regulator housing preferably takes the form of a sleeve. It will be appreciated that the regulator housing may include two or more housing parts secured together, or may be a unitary part.
Preferably, the position of the stop member relative to the regulator housing is adjustable to permit adjustment of a pre-set fluid pressure at which the abutment member is caused to move into engagement with the stop member.
When fluid pressure acting on the piston member exceeds the pre-set fluid pressure, the piston member is moved, against the biasing force due to the biasing arrangement, through a sufficient distance to cause the abutment member to engage the stop member. During this stage of operation, the rate at which displacement of the piston member varies with fluid pressure applied to the piston member is determined by the characteristics of both the first and second spring means. When the moveable abutment member moves into engagement with the stop member movement of the abutment member is terminated, the second spring means is disabled and the rate at which displacement of the piston member varies with fluid pressure is altered. During this stage of operation, the rate at which displacement of the piston member varies with fluid pressure applied to the piston member is determined by the characteristic of only the first spring means.
The predetermined fluid pressure at which the moveable abutment member moves into engagement with the stop member can be adjusted by adjusting the position of the stop member relative to the regulator housing.
When the pressure regulator is used in regulating the outlet pressure of a transfer pump for use in a fuel system, the speed of the associated engine is related to the pressure of fuel acting on the piston member. Hence, by adjusting the position of the stop member, the engine speed at which the moveable abutment member is caused to move into engagement with the stop member can be adjusted. The pressure regulator can therefore be adjusted to give the required transfer pressure-engine speed characteristic.
The invention provides the advantage that, as the pressure-speed characteristic of the pump can be adjusted, pump to pump variations can be compensated for. The pressure-speed characteristic of the pump can also be selected so as to match the requirements of the particular engine.
Preferably, the stop member takes the form of an adjustment screw.
The pressure regulator preferably comprises an adjustment member for adjusting the pre-load of the biasing arrangement. Preferably, the adjustment member takes the form of a further adjustment screw.
Conveniently, at least a part of the stop member extends through a further bore provided in the adjustment member.
The moveable abutment member conveniently includes an annular region defining first and second abutment surfaces, the abutment member being arranged such that the first abutment surface is in abutment with one end of the first spring means and the second abutment surface is in abutment with one end of the second spring means.
Preferably, the piston member itself is arranged to obscure the outlet opening, rather than any component carried by or otherwise coupled to the piston member.
Preferably, the piston member shaped to cooperate with the outlet opening so as to vary the extent to which the outlet opening is obscured, thereby to provide progressive opening of the outlet opening depending upon the extent to which the piston member is displaced.
Preferably, the piston member is of generally cylindrical form and has a generally cylindrical outer surface which cooperates with the outlet opening to vary the extent to which the outlet opening is obscured, depending on the position of the piston member within the bore.
The diameter of the piston member and the diameter of the bore are preferably selected to ensure movement of the piston member within the bore is guided.
In a preferred embodiment, the piston member is exposed to fuel pressure within an inlet chamber defined by the bore within which the piston moves.
In a further preferred embodiment, the piston member is arranged such that it can completely obscure the outlet opening to prevent fuel flow from the inlet chamber through the outlet opening.
The pressure regulator provides a continuous pressure regulating function as the piston is urged to move against the spring load upon increasing fuel pressure within the inlet chamber.
According to a second aspect of the present invention, a transfer pump for use in supplying fuel to a high pressure pump for an engine comprises a pump outlet through which fuel is supplied at a pressure dependent upon engine speed to the high pressure pump, the transfer pump comprising a pressure regulator as herein described which is arranged to regulate the pressure of fuel delivered by the transfer pump to the high pressure pump, in use.
In a preferred embodiment, the pressure regulator of the transfer pump comprises an inlet region defined by the bore within which the piston member of the pressure regulator moves to vary the extent to which the outlet opening is obscured, the pump outlet being arranged to deliver fuel to the inlet region at a pressure dependent upon the engine speed.
It will be appreciated that the second aspect of the invention may include any one or more of the preferred or optional features of the first aspect of the invention.
According to a further aspect of the present invention, a fuel system for supplying fuel to an engine comprises a transfer pump for delivering fuel at relatively low, regulated pressure to a high pressure fuel pump, the high pressure fuel pump being arranged to deliver fuel to the engine, wherein the fuel system comprises a pressure regulator as herein described which is arranged to regulate the pressure of fuel delivered by the transfer pump to the high pressure pump, in use.
It will be appreciated that the further aspect of the invention may include any one or more of the preferred or optional features of the first aspect of the invention.